The present invention relates to high-frequency inverters in general, and more particularly to the technique of starting a high-frequency inverter, typically for induction heating applications.
An induction heating apparatus basically includes a thyristor-controlled bridge rectifier for converting the industrial AC power supply (typically 440 V, 3-phase 60 Hz) to a controllable DC voltage which is in turn applied to a thyristor inverter generating high frequency AC power to a work coil tuned with a capacitor.
For induction heating applications, the work coil load itself is predominantly inductive, with loaded Q-factors in the range between 2.0 and 20. Thus, it is necessary to "tune out" the inductive kVA of the work coil with a capacitor in order to enable the inverter to operate at, or near to, unity power factor. The compensating capacitor is connected in parallel across the load, while the load includes a parallel-tuned load at a slightly higher frequency than the resonant frequency of the load. The inverter is of the current-fed type, e.g., developing an essentially square wave current into the parallel-tuned load. Therefore, the load phase angle, as seen by the inverter, is always slightly leading and the output current leads the output voltage by an angle which is sufficient to ensure that, in the course of successive gating of the thryistors of the inverter, the load voltage commutates the current from one pair of thyristors to the other, before reversing its polarity.
Due to the relatively high Q-factor of the work coil, the parallel-tuned load circuit is necessarily under-damped, e.g., oscillatory. This leads to the square wave of current generating a sinusoidal voltage across the load.
As generally known with the current-fed inverter, the square wave of current at the output is "forced" by the presence of a relatively large smoothing inductance on the side of the DC link voltage. This inductor also insures that the input current has only a relatively small superposed AC ripple component.
A serious disadvantage with a current-fed inverter for energizing a parallel-tuned load is the difficulty of starting the inverter. Before the thyristors of the inverter can be cycled safely for successive gating and commutating at the required frequency of the load, first the line inductor should have received from the rectifier power supply sufficient current flow, and the parallel-tuned load should have enough energy available for commutation of the thyristors initially.
As explained in U.S. Pat. No. 3,599,078 of Brian R. Pelly, auxiliary means have been proposed, such as a parallel-tuned inverter controlled at a lower frequency which is charged, then, discharged into the load to provide initial energy at start-up. However, the starting techniques have not been satisfactory when, as in induction heating, the main inverter is required to operate over a wide frequency range to accommodate a wide range of loads. Difficulty also arises which is due to the stray capacitance associated with the leads connecting the tuned load to the inverter, especially at high frequency of operation. Alternate techniques to the one disclosed in the aforementioned Pelly patent, which have been used previously to start the inverter with its parallel-tuned load, have been to (1) initially disconnect the output tuning capacitor from the inductance coil; (2) to precharge the inductance coil with a precharging current which is then diverted through the inverter into the tuned load circuit to shock the load into oscillation. For reasons explained in the Pelly patent, these prior art approaches are not satisfactory.
The "parallel compensated" inverter circuit for induction heating applications is required to operate in a high frequency range, typically up to 9.6 kHz. At such high frequency, the starting circuit becomes critical, especially when phased out into the free-running operation of the inverter, since the main thyristors must be able to be safely commutated by that time. Accordingly, it has been proposed to modify the circuit by adding a capacitor in series with the load for starting, then to disconnect the capacitor once the inverter is running. As a further development, it has been conceived that if such capacitor could be left permanently in circuit, the auxiliary start-circuit could be advantageously simplified. This is especially a warranted conclusion, since the series capacitor actually makes a useful contribution in the normal operation of the induction heating apparatus by correcting the overall load phase angle, as viewed by the inverter, towards a desirable leading phase angle. Accordingly, the parallel capacitive compensation required for the load can be reduced. In other words, the presence of a series capacitor enables the parallel compensated load to operate at a better power factor than otherwise would be the case. Moreover, with a transformer-coupled load, as commonly used in order to provide isolation from the workpiece side, the presence of such series capacitor is of a definite practical advantage by reducing the required kVa rating of the output transformer.
It is also observed that, when rated power is being delivered to the load with the series capacitor, namely with a "series/parallel compensated" output circuit, there is no limiting minimum critical value of output load impedance, and the inverter is capable of supplying the full rated output current, at any level of output voltages. See: P. Knapp, "Characteristics of the Parallel-Resonance Inverter for Inductive Melting," Brown-Boveri Review, October, 1966.